Liquid crystal display devices typically operate by the application of an electric field to liquid crystal material which causes polarization of the liquid crystals, and assumption of a light-modulating characteristic (for example, light-transmissivity).
Devices of the prior art commonly employ spaced conductive lattice networks, single conductive planes with spaced conductive lattice networks, or pairs of conductive planes as the means to apply the electric field. Such displays typically operate to switch liquid crystal material in predetermined regions to display numerals, figures, and the like, which themselves are in fixed positions. In such cases, electrical potential typically is applied to all of a predetermined region, rather than to a point, line or area determined only by the shape of a stylus or other free-hand applicator implement.
Among prior liquid crystal devices are some with certain slate-like characteristics, including some of those disclosed in the following patents: U.S. Pat. Nos. 4,814,760 (Johnston et al.); 3,781,085 (Liebowitz); 4,685,770 (Baeger); 4,525,032 (Hilsum); 3,947,183 (Haas et al.); 4,723,836 (Kono et al.); 4,527,862 (Arakawa); 3,886,311 (Rodger et al.); 4,643,528 (Bell, Jr.); 4,688,900 (Doane et al.); 4,433,900 (Sekimura); and 4,221,471 (Gurtler).
Some of such devices are equipped with writing instruments intended to change the polarization of a liquid crystal. However, these and other liquid crystal devices of the prior art are typically limited in the sense that images usually cannot be formed at virtually any location on image-displaying surface areas.
Liquid crystal devices of the prior art have a number of shortcomings and/or disadvantages when considered as writing/drawing slates for various purposes. Since AC potential is usually applied in liquid crystal devices, images formed in free-hand slate devices would tend to be of uneven intensity. This is especially the case if an imaging implement is drawn across a slate-like display at a rapid rate, particularly if the applied AC voltage is at a low frequency (for example, 60 Hz).
One possible approach to overcoming this "zebra striping" effect would involve the use of very expensive AC power supplies having output frequencies of several thousand Hz. This can be impractical for a variety of desired liquid crystal slate applications.
Those prior liquid crystal displays which apply direct current voltage apply such voltage to a specific predetermined area, as noted above, and typically at a low voltage. In such cases, there is no need to consider the effect of applied DC voltage on the typically transparent conductive layer (or lattice or symbol, as the case may be). In particular, there is no need for concern about a concentrated direct application of relatively higher DC voltage on a very small area, such as would be applied from the tip of a charged stylus.
In fact, it is known that relatively higher DC voltage directly applied in free-style fashion to a transparent conductive layer by a stylus or the like may degrade the laminate. More specifically, degradation could occur to the transparent very thin conductive layer of metallized film or the like (for example, indium tin oxide). It is the typical sandwich-like characteristic of liquid crystal displays (specifically, the presence of a transparent second conductive layer) that imposes this problem.
A low voltage is typically applied directly to a liquid crystal area by the usual sandwich-like spaced conductive layers (one of which is a transparent thin conductive layer like ITO). It can be appreciated that it would be necessary to apply a significantly higher voltage to polarize a liquid crystal material if the liquid crystal were dispersed and held in a surrounding polymer and/or otherwise separated from a voltage-applying member by non-conductive material.
Liquid crystal devices of the prior art which have some slate-like characteristics tend to be fairly complex and expensive. Liquid crystal technology has not readily lent itself to use in writing/drawing slates, particularly to devices having wide application for writing/drawing uses, classroom uses, toy or game uses, or the like. The prior art indicates a failure to appreciate that an applied voltage which is higher than that typically applied in liquid crystal devices can be used to provide significantly improved liquid crystal imaging. The prior art also fails to appreciate the variety of ways in which such voltage can be extended to an image-displaying surface area.
A variety of light-modulating liquid crystal products have been made utilizing LC-layers which have a polymeric material forming and holding micro-volumes of liquid crystal material. However, LC-layers of this type have heretofore not been successfully utilized in simple slate products of the writing/drawing type.
A liquid crystal display having a laminate of which one surface is substantially conductor-free and which includes means for selective transitory application of DC electrical potential to form an image of uniform intensity independent of application method and without degradation of the laminate would be an important advance in the art.